MAMMALIA. 463 



supremacy to development of the mental faculties. The 

 brain develops in a typical vertebrate manner, and we may 

 here merely note the following characteristics : — 



1. The cerebral hemispheres are large and encroach 

 backwards over the thalamencephalon and the optic lobes. 

 In the higher types their surface becomes much convoluted 

 and they cover the cerebellum. 



2. The cerebral hemispheres are united across the 

 middle line by the corpus callosiim. 



3. The optic lobes become divided to form four, the 

 corpora quadrigemina. 



4. One of the most striking characters of the mammalian 

 brain is the great increase in proportionate size that has 

 taken place in comparison with the brain of extinct forms. 

 The brain of the Eocene mammals was far smaller in pro- 

 portion to the total bulk than that of modern forms. This 

 is probably due to the fact that since that epoch the race 

 has not been so much to the strong as to the *' cunning." 



In the same way, if we compare the weight of a mammal's brain 

 with the total weight of the body, we find that there are three impor- 

 tant laws. 



Firstly, in equally organised animals the relative weight of brain 

 decreases with increase in size. Thus the smallest animals tend to have 

 proportionately heavier brains. The relative brain-weight of a cat is 

 given as y^, whereas that of a tiger is -g-^^. On account of this law, 

 we find that the relative brain-weight of man (^V) is exceeded by that 

 of the lesser shrew {-^q) and the whiskered bat (^V)* 



Again, the relative brain-weight increases very rapidly in proportion 

 to the organisation of the animal and in animals of equal size it varies 

 with the organisation. 



Thus we may cite from Dubois the following equal-sized species: — 



Siamang (Simiidse) \ p ^-.^..^^ / tV 



Budlug(Cercopithecid^)/ ^^^i^^^^^es, | ^ 



Civet-Cat, Carnivora, yw 



Javan Pangolin, Edentata, 753-5 



If the effect of the varying size of mammals be eliminated, a table 

 showing degree of " cephalisation " can be formed, and this agrees 

 generally with the recognised succession of the mammalian orders, the 

 Metatheria, Edentata, Rodents and Insectivora taking the lowest 

 places, followed by Ungulata, Cetacea, Carnivora and lower monkeys, 

 and, lastly, anthropoid apes and man. 



Thirdly, taking extinct mammals into account, it would appear that 

 in mammals of similar size and bodily organisation the relative brain- 

 weight increases with the time, as we have seen that the greatest 

 advance from Eocene times has been cerebral. 



